1. Field of the Invention
The present invention relates to a liquid crystal display and particularly to a fast multistable liquid crystal display using a cholesteric liquid crystal material.
2. Description of the Prior Art
Recent concerted efforts in the field of liquid crystal materials have yielded a new class of reflective, cholesteric texture materials and devices. These liquid crystal materials have a periodic modulated optical structure that reflects light. The liquid crystal material comprises a nematic liquid crystal having positive dielectric anisotropy and chiral dopants. These materials are known as polymer stabilized cholesteric texture (PSCT) and polymer free cholesteric texture (PFCT).
LCDs using the reflective cholesteric texture liquid crystal are known as fast multistable LCDs (FMLCD). The reflective cholesteric texture liquid crystal (both PSCT and PFCT) has two stable states at a zero applied field. One such state is the planar texture state which reflects light at a pre-selected wavelength determined by the pitch of the cholesteric liquid crystal material itself. The other state is the focal conic texture state substantially optically transparent. By stable, it is meant that once set to one state or the other, the material will remain in that state, without the further application of an electric field. Conversely, for other types of conventional LCDs, each liquid crystal picture element must be addressed many times each second in order to maintain the information stored thereon. Accordingly, PSCT and PFCT materials are highly desirable for low energy consumption applications, since once set they remain so.
The configuration of FMLCDs is substantially the same as in conventional passive LCDs: picture elements (pixels) are addressed by crossing lines of transparent conducting lines known as common and segment lines respectively formed on two substrates with the cholesteric liquid crystal sealed therebetween. Conventional methods for addressing or driving such displays can be understood from FIG. 1. FIG. 1 illustrates a table showing the state of the liquid crystal material after the application of various driving voltages thereto. The liquid crystal material begins in a first state, either the reflecting state or the transparent state, and is driven with an AC voltage, having an amplitude above V4 in FIG. 1. When the voltage is removed quickly, the liquid crystal material switches to the reflecting state and remains there. If driven with an AC voltage between V2 and V3 the material will switch into the transparent state and remain until the application of a second driving voltage. If no voltage is applied, or the voltage is well below V1, then the material will not change state, regardless of the initial state.
The conventional single polar driving method of FMLCDs is described in the following accompanied by FIGS. 2A-2D. In the single polar driving method, it should be noted that all the voltage levels applied to the segment and common lines have the same polarity (positive, for example) based on the ground voltage (0V, for example), and the waveforms of the differential voltage signal between the common and segment line in each pixel are centered at the ground voltage. For a given single pixel, the differential voltage signal Vpixel between its common and segment line may have four different amplitudes during different periods.
As shown in FIG. 2A, in an addressing period when the pixel is unselected (beyond its scan period) and a binary bit “H” is sent to the pixel, a square wave switching between 5.5V and 24.5V is applied to its common line and another square wave switching between 0V and 30V is applied to its segment line. The two square waves are in phase. Thus, the differential signal Vpixel switches between −5.5V and 5.5V, and has an amplitude of 11V designed to be lower than V1. The result is no change in the pixel.
As shown in FIG. 2B, in an addressing period when the pixel is unselected and a binary bit “L” is sent to the pixel, the square wave switching between 5.5V and 24.5V is applied to its common line and another square wave switching between 11V and 19V is applied to its segment line. The two square waves are in phase. Thus, the differential signal Vpixel switches between 5.5V and −5.5V, and has an amplitude of 11V lower than V1. The result is no change in the pixel. Those skilled in the art will appreciate that the unselected rows of the pixels (beyond the scan period) must be kept unchanged despite the data bits received.
As shown in FIG. 2C, in an addressing period when the pixel is selected (in its scan period) and the binary bit “H” is sent to the pixel, a square wave switching between 30V and 0V is applied to its common line and the square wave switching between 0V and 30V is applied to its segment line. The two square waves are out of phase (have a phase difference of 180°). Thus, the differential signal Vpixel switches between −30V and 30V, and has an amplitude of 60V designed to be higher than V4. As a result, the pixel is driven to the reflecting state as shown in FIG. 1.
As shown in FIG. 2D, in an addressing period when the pixel is selected and the binary bit “L” is sent to the pixel, the square wave switching between 30V and 0V is applied to its common line and the square wave switching between 11V and 19V is applied to its segment line. The two square waves are out of phase (have a phase difference of 180°). Thus, the differential signal Vpixel switches between −19V and 19V, and has an amplitude of 38V designed to be located between V2 and V3. As a result, the pixel is driven to the transparent state as shown in FIG. 1.
Further, all pixels should be erased before addressing by the signals on the segment and common lines shown in FIGS. 2A-2D, by driving the pixels into reflecting state. Thus, in the erasing period, square waves having a phase difference of 180° with the square waves on the segment and common line as shown in FIG. 2C are applied to all pixels.
However, in the conventional single polar driving method, the common and segment drivers must be capable of providing tour different positive voltage levels. The design of the drivers with 4-level output is complicated and a relatively large number of power supplies must be used.